1. Field of the Invention
The present invention relates generally to utility pliers and, more particularly to self-adjusting pliers that self-adjust for grasping a workpiece and/or self-locking pliers for automatically locking the pliers into engagement with the workpiece.
2. Description of the Prior Art
Many types of handheld utility pliers are known in the art. Conventional pliers typically include two plier members interconnected in a scissor-like arrangement allowing for a workpiece to be grasped by jaw portions of the pliers in response to movement of handle portions of the pliers. Over the years, numerous improvements have been made to the conventional plier design in order to obtain a better and more efficient plier. For example, self-adjusting pliers have been developed in order to provide a set of pliers that more easily adjust to the size of a given workpiece. In addition, self-locking pliers have been developed in order to provide a set of pliers that will allow the pliers to remain in locking engagement with the workpiece.
U.S. Pat. No. 4,651,598 discloses a self-adjusting utility plier. These pliers provide for self-adjustment through employment of a spring-biased control arm positioned between the handles. During the self-adjustment, the pivot point of the pliers lies in the spring-biased control arm. As can be appreciated, the location of the pivot point is continuously changing until such time as the workpiece is engaged resulting in the center of the pawl member becoming the main pivot point. A disadvantage of such an arrangement is the lack of controlled, rotating movement inherent in the arrangement of the continuously changing initial pivot point. This produces slack in the pliers which may lead to unpredictable pawl engagement between the pawl member and cooperating rack. Another disadvantage of the compound movement inherent in the control arm linkage and its continuously changing location of the pivot point, as opposed to a simple pivoting movement around a fixed pivot point, is the requirement for generous clearance between the moving parts of the pliers. Consequently, when the teeth of the pawls and the mating teeth come into engagement as a result of clamping force applied to the handles, a different set of teeth may become engaged at different times in essentially identical clamping operations resulting in unpredictable clamping performance. Consequently, excessive slack within the linkage-based mechanism has to be taken up each time an object is grasped and a significant portion of the handle movement is dissipated before clamping forces are being applied to the workpiece. This limits the separation of the jaws for a given handle spacing, limiting the size of object that can be grasped. Conversely, to achieve a larger jaw opening for grasping larger objects, the handle spacing becomes excessive for one-handed operation of the pliers. In addition, the main pivot point is located inside of a pawl which is itself inside of an elongated channel or slot. A disadvantage of this arrangement is that this limits how close the main pivot point can be located to the jaws and thus, the ultimate leverage that can applied by squeezing the handles is limited. Similar self-adjusting utility pliers are disclosed in U.S. Pat. Nos. 4,662,252, 4,802,390, 4,893,530, 5,060,543 and 5,351,584.
U.S. Pat. No. 5,140,876 discloses variable-fulcrum pliers. The pliers initially pivot about a fixed pivot pin until the jaws of the pliers grip a solid object. The jaws then become the fulcrum which causes the pivot point to shift to a fixed pivot lug which acts as the fulcrum of the pliers as additional pressure is applied to the handle portions of the pliers. In this arrangement, the two pivot points are aligned transversely to the longitudinal axis of the pliers. A disadvantage of this arrangement is that there is not rapid self-adjustment with minimum handle movement during the adjustment cycle. A further disadvantage is that by transversely aligning the pivot points with respect to the longitudinal axis of the pliers, the maximum leverage during the clamping cycle is not obtained.
As to self-locking pliers, the most common self-locking plier is the well-known VISE-GRIP pliers. VISE-GRIP pliers employ a toggle mechanism which allows for the jaws to be maintained in locking engagement with a workpiece once a force is applied to the handle members of the pliers. Typically, VISE-GRIP pliers include an adjustment screw which must be initially adjusted to set the opening of the jaws in relation to the workpiece to be grasped. As can be appreciated, operation of the VISE-GRIP pliers requires several steps and further requires that the user employ both hands to operate the same. Examples of the well-known VISE-GRIP pliers are disclosed in U.S. Pat. Nos. 3,354,759, 3,496,808, 5,056,385 and 5,435,214.
Still other pliers have been developed which include a combination of the self-adjusting and self-locking features. Such pliers are disclosed in, for example, U.S. Pat. Nos. 1,772,428, 1,944,116 and 2,620,697. The pliers disclosed in these patents utilize an arcuate arm positioned between the handle members of the pliers and employ a clutch arrangement or friction arrangement for adjusting and/or locking the pliers as the handle members are moved toward each other. These type pliers do not provide for a wide range of self-adjustment and the mechanisms employed therein are difficult to operate and require both hands of the user to for operation.
There remains a need for improved self-adjusting and/or self-locking handheld utility pliers which provide a greater mechanical advantage than previously known pliers and which can be easily operated by the user, preferably with one hand.